1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device of an OCB (Optically Compensated Bend) mode, which can realize a wide viewing angle and high responsivity.
2. Description of the Related Art
A liquid crystal display device has various features such as thickness in size, light weight, and low power consumption. The liquid crystal display device is applied to various uses, e.g. OA equipment, information terminals, timepieces, and TVs. In particular, a liquid crystal display device comprising thin-film transistors (TFTs) has high responsivity and, therefore, it is used as a monitor of a mobile TV, a computer, etc., which displays a great deal of information.
In recent years, with an increase in quantity of information, there has been a strong demand for higher image definition and higher display speed. Of these, the higher image definition is realized, for example, by making finer the array structure of the TFTs.
On the other hand, in order to increase the display speed, consideration has been given to, in place of conventional display modes, an OCB (Optically Compensated Birefringence) mode, a VAN (Vertically Aligned Nematic) mode, a HAN (Hybrid Aligned Nematic) mode and a π alignment mode, which use nematic liquid crystals, and an SSFLC (Surface-Stabilized Ferroelectric Liquid Crystal) mode and an AFLC (Anti-Ferroelectric Liquid Crystal) mode, which use smectic liquid crystals.
Of these display modes, attention has been paid to an OCB mode liquid crystal display device as a liquid crystal display device that can improve the viewing angle and the responsivity. In the OCB mode liquid crystal display device, a liquid crystal layer that includes liquid crystal molecules, which can be aligned with a bend, is held between a pair of substrates. Compared to the TN mode liquid crystal display device, the OCB mode liquid crystal display device has an improved responsivity that is higher by an order of magnitude. In addition, the OCB mode liquid crystal display device advantageously has a wider viewing angle since the effect of birefringence of light, which passes through the liquid crystal layer, is optically self-compensated by the alignment state of liquid crystal molecules.
In the case where an image is displayed by the OCB mode liquid crystal display device, black may be displayed by blocking light at a time of, e.g. high voltage application and white may be displayed by passing light at a time of low voltage application, with the control of birefringence and in combination with a polarizer plate.
When a black image is displayed, a majority of liquid crystal molecules are aligned in an electric-field direction by the high voltage application (i.e. aligned in a normal direction to the substrates). However, liquid crystal molecules in the vicinity of the substrates are not aligned in the normal direction due to interactions with the orientation films. Consequently, light that travels through the liquid crystal layer is affected by a phase difference in a predetermined direction. Owing to the effect of phase difference (retardation), in the case of viewing the screen from a frontal direction (i.e. in the normal direction to the substrate), the transmittance cannot sufficiently be reduced when a black image is displayed, and the contract deteriorates.
To cope with this problem, a uniaxial phase plate, for instance, may be used in combination. Thereby, the retardation of the liquid crystal layer is compensated when a black image is displayed, and the transmittance can sufficiently be reduced, as is conventionally known. Besides, as disclosed in Patent Document 1, for instance, it is known that discotic liquid crystal layers including hybrid-aligned optically negative discotic liquid crystal molecules are combined as phase plates, whereby black display with a sufficiently low transmittance is realized or contrast characteristics are compensated even when the screen is obliquely viewed (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 10-197862).
In the conventional structure of the OCB mode liquid crystal display device, a bend liquid crystal layer comprising bend-aligned liquid crystal molecules is disposed between two polarizer plates having absorption axes (or transmission axes) intersecting at right angles. In addition, in order to compensate viewing-angle characteristics at a time of black display, two discotic liquid crystal layers and two biaxial phase plates are used.
In this conventional structure, there are three objects for optical compensation: a positive retardation in a normal direction to the bend liquid crystal layer; a residual retardation in an in-plane direction of the bend liquid crystal layer; and positive dichroism of the polarizer plate.
Specifically, the bend liquid crystal layer, when viewed as a whole, is a biaxial refractive-index anisotropic body (nz>nx>ny). Hence, the bend liquid crystal layer has a positive retardation in its normal direction, and has retardation in its in-plane direction. The retardation in the normal direction is mainly compensated by using the discotic liquid crystal layer and biaxial phase plate. The residual retardation in the in-plane direction is mainly compensated by using the discotic liquid crystal layer.
However, these retardations that are due to a liquid crystal have a large degree of wavelength dispersion. On the other hand, in many cases, the biaxial phase plate for compensating the retardation in the normal direction is formed of a drawn film, and the wavelength dispersion thereof cannot be increased to a level of the phase plate including liquid crystal molecules. Thus, even if the retardation in the normal direction is successfully compensated with respect to a specific wavelength, the wavelength dispersion of the bend liquid crystal layer does not agree with the wavelength dispersion of the biaxial phase plate with respect to the other majority of wavelengths, and adequate compensation cannot be effected. Consequently, there arises such a problem that the contrast characteristics and color reproducibility in relation to the inclination from the normal of the screen, that is, the viewing-angle characteristics of contrast and color, are insufficient.
Like the bend liquid crystal layer, the discotic liquid crystal layer for compensating the in-plane directional retardation has a large degree of wavelength dispersion, and the retardation in the in-plane direction of the bend liquid crystal layer can be almost compensated in the range of visible wavelengths. Thus, the contrast characteristics and color reproducibility in the normal direction of the screen are good.
The positive dichroism in the polarizer plate can be compensated by using the biaxial phase plate having a slow axis in a direction intersecting at right angles with the direction of dichroism. However, the dichroism of the polarizer plate has a wavelength dispersion with a polarity opposite to the polarity of the wavelength dispersion of the bend liquid crystal layer (for example, the bend liquid crystal layer has such wavelength dispersion that the retardation increases as the wavelength becomes shorter, while the polarizer plate has such wavelength dispersion that the retardation increases as the wavelength becomes greater). By contrast, the biaxial phase plate has a low degree of wavelength dispersion. Thus, even if the positive dichroism is successfully compensated with respect to a specific wavelength, the wavelength dispersion of the polarizer plate does not agree with the wavelength dispersion of the biaxial phase plate with respect to the other majority of wavelengths, and adequate compensation cannot be effected. Consequently, there arises such a problem that the viewing-angle characteristics of contrast and color are insufficient.
In addition to these problems, there is such a problem that the manufacturing cost of the discotic liquid crystal layer and biaxial phase plate is high, and this leads to an increase in manufacturing cost of the whole liquid crystal display apparatus.